1. Field of the Invention
The present invention relates to underground drainfield systems and methods for treating, filtering, cleaning, discharging and recycling septic tank effluent or drainfield water in difficult soil conditions that historically have required an above ground mound system or an intermittent sand filter.
2. The Prior Art
In the United States, a growing number of households rely upon a septic system rather than centralized wastewater treatment facilities. In fact, approximately one fourth of households in the United States use a septic system to treat, filter, clean and disburse wastewater. A typical septic system consists of a septic tank, a distribution/filtration box and some form of an underground disposal field. Several types of underground disposal fields have been developed and are known in the art. The most common type is a drainfield, also known as a leach field or absorption field. There have been several variations of drainfields, including mound systems, sand filters and dig outs.
Once sewage undergoes treatment in a septic tank, the resulting effluent is transported to the drainfield. This is accomplished by either gravity or through a mechanical pump, with the goal of uniformly discharging effluent below ground into the soil for final treatment and disposal. Another goal of the drainfield is to naturally filter the post-septic tank effluent to remove any remaining pathogens, bacteria, or biomass prior to flowing into the ground water. Sizing of a drainfield depends upon several factors including the area of the property, the number of individuals in the household, water usage habits of the household, on-site soil conditions, and government regulations. One typical form of a drainfield comprises a collection of multiple parallel-perforated pipes connected by one or more distribution pipes that allow distribution of effluent into the surrounding ground soil for filtration.
Historically, construction of a drainfield has been expensive, time consuming and inconvenient. Construction usually begins with the excavation of a large rectangular cross section of land by digging multiple trenches at least three feet deep to lay the necessary perforated pipes. These trenches are usually less than 100 feet long, and dug to create an essentially flat bottom. In one prior art drainfield construction, each trench is first filled with a layer of gravel. Next, a perforated pipe is placed in the trench, with an additional six-inch layer of gravel added to surround the perforated pipe. A geotextile fabric or a similar product is placed over the approximately two feet of gravel. Finally, a covering layer of backfill soil is added. This entire process requires transport of large amounts of gravel, backfill soil and piping from a distribution center to the drainfield site. The steps of digging trenches, creating a network of piping and laying different layers of filtering media requires specialized equipment, multiple experienced workers, time, and large amounts of natural resources.
In many areas of the country, unique soil conditions require a modified drainfield known as a mound or raised drainfield. In areas with high groundwater, shallow soil over impermeable soil or slowly permeable soil, a mound must be created above ground to allow proper distribution and filtration of post-septic effluent. However, above ground mounds often require a mechanical pump to raise effluent from the septic tank above ground to the mound. Second, mounds require transport of additional natural resources to the site. Third, mounds are typically unsightly and greatly reduce the use of the land. Lastly, mounds require a relatively larger area than conventional drainfields and also require routine monitoring and maintenance.
For areas with high ground water or impervious soil, one alternative for a mound or raised drainfield is an intermittent sand filter. An intermittent sand filter is a water impermeable basin placed in the ground containing a network of perforated pipes located in a sand bed. The water impermeable basin is first filled with a layer of aggregate, most commonly pea gravel. Next, a second layer of medium grade clean sand is added to the basin to create the sand bed. A network of perforated pipes is placed on top of the sand bed. A second layer of aggregate is then added to the basin. A larger perforated outflow pipe is typically placed within the basin for collection of filtered effluent that then enters the drainfield.
Although intermittent sand filters reduce the need for a mound, improve the appearance of the underground disposal field and allow for better use of the ground, there are several disadvantages. First, intermittent sand filters require transporting large volumes of heavy sand to the drainfield site, which can be very costly. Second, intermittent sand filters require very large cross sections to be effective. For example, a typical two-bedroom home would require a sand filter nineteen by nineteen feet in cross section. Thus, these systems can only work with large acreage households. Third, most intermittent sand filters require a mechanical pump, which results in greater energy and maintenance costs.
Apart from intermittent sand filters and mound systems, a third type of drainfield called a “dig out” has also been used in the art. With a dig out, a large cross sectional area of the soil near the septic tank is excavated to remove poor soil. Good quality soil is then transported to the site through one or more commercial vehicles. The good soil is then evenly deposited within the excavated area. A network of perforated piping is assembled and placed on top of the newly deposited soil, which is connected to either a distribution box or directly to the septic tank. Backfill soil is then added over the network of perforated piping. While this method of creating a drainfield has some benefits with respect to an intermittent sand filter, the overall costs, manpower and natural resources required to create a dig out system are significantly greater.
There exists a need for an alternative to intermittent sand filters and mound drainfields for efficient treatment, filtration, and distribution of effluent. In addition, there is a need for a filter media that is light weight, portable, inexpensive and allows for increased filtration to decrease the cross sectional size of these systems. Finally, there is a need for such systems to be modular for easy transport to the drainfield site, to allow improved fabrication of these systems, for further reduction in overall costs.